A typical cathode ray tube (CRT) scans a variable-strength electron beam over a phosphor (solid material which luminesce when suitably excited, such as silver-activated zinc sulfide and silver-activated zinc cadmium sulfide) screen, thereby producing a visible image through luminescence of the excited phosphor. In a color television, the luminance transmission is supplemented by the chrominance transmission which, in cooperation with a shadow mask, produces images in full color. The shadow mask is a perforated plate that contains about 200,000 precisely located holes and lies directly behind the phosphor screen. Each of the holes is accurately aligned with three different colored phosphor dots that make up a single pixel of the screen, the screen being composed of a multitude of such pixels evenly distributed throughout. When properly aligned, the shadow mask enables the visible images to have crisp pure colors and sharp well-defined edges. It accomplishes this by limiting the area that the electron beam can strike. The three electron guns inside the CRT generate a triplet of electron beams, each for a different color. Each beam must hit only the part of the pixel containing the phosphor that corresponds to its color. The shadow mask acts as a pinhole lens, using parallax due to the offset of the beams to allow the correct beam to hit the correct phosphor dot.
Monochrome CRT's have 640.times.480 pixels (NTSC standard) and HDTV (high definition television) contains 1125.times.768 while computer monitors have 1280.times.1024 pixels. As the number of pixels increases, the shadow mask's manufacturing requirements and alignment tolerances become extreme. If, due to material stresses, thermal expansion, assembly line quality control problems, mechanical shock or sundry other problems, the mask is misaligned by even one half of a pixel size, the overall image intensity and the picture sharpness and colorimetry can be significantly degraded over a substantial area of the screen. In order to have and maintain superior image quality, the shadow mask must be correctly located to within a small fraction of a single pixel and must remain positioned so for the life of the image tube.
Therefore, any technique or device which provides the same function as the shadow mask but with less stringent tolerance requirements and less sensitivity to misalignment will improve the image technology and yield a more rugged, durable image tubes without the high cost of extreme mechanical tolerance requirements. Also, the removal of the shadow mask will alleviate the requirement for exact focusing of the electron beam which is now necessary to obtain adequate screen brightness after losses due to the shadow mask.